Integrated circuits frequently incorporate devices that extend deeply into a semiconductor substrate or wafer. In the manufacture of such integrated circuits, it is desirable to periodically inspect the structure of these devices. In most cases, this is achieved by inspecting a cross-section of the wafer on which the integrated circuits are formed.
The conventional method for inspecting a cross-section of a semiconductor substrate generally includes the removal of material from the substrate in order to expose a cross-sectional surface. This is followed by the illumination of that surface by an imaging beam. The step of exposing a cross-sectional surface is typically accomplished by scanning the front surface of the semiconductor substrate with a focused ion-beam to excavate a trench. The vertical wall of this trench forms the cross-sectional surface to be imaged by the imaging beam.
The imaging beam is typically a particle beam that scans the cross-sectional surface exposed by the scanning focused ion-beam. The interaction of this particle beam with the cross-sectional surface results in the emission of charged particles. These charged particles are detected by a detector that provides data to a processor for transformation of the data into a cross-sectional image of the semiconductor substrate.
A disadvantage of the foregoing method of obtaining a cross-sectional image is that the cross-sectional surface is exposed point by point using a focused ion-beam. Because the ion-beam is focused to a small point, the source of ions must be one that is capable of providing a large ion current in a small area. Such sources, which are referred to as “bright” sources, typically use, as a source of ions, a metal that is liquid at or slightly above room temperature. For brevity, we refer to such metals as “liquid metals.” Of these liquid metals, the one generally considered most practical, because of its high boiling point, is gallium.
Unfortunately, it is extremely undesirable to introduce stray metal ions, such as gallium, into the semiconductor fabrication process. The presence of even small amounts of metal ion left on a semiconductor substrate following exposure to a focused ion beam can, through diffusion, contaminate other circuits formed on that substrate. As a result, following inspection of the cross-section of a wafer, the entire wafer is routinely discarded.
It is therefore desirable in the art to provide a method and system for imaging a cross-section of a substrate but without the deposition of stray metal ions on the substrate.